Field of the Invention
The present invention relates to a novel stimulable phosphor, a process for the preparation of the same, a radiation image recording and reproducing method utilizing the same, and a radiation image storage panel employing the same. More particularly, the invention relates to a bismuth activated alkali metal halide stimulable phosphor.
Description of the Prior Art
For obtaining a radiation image, there has been conventionally employed a radiography utilizing a combination of a radiographic film having an emulsion layer containing a photosensitive silver salt and an intensifying screen.
As a method replacing the above-described radiography, a radiation image recording and reproducing method utilizing a stimulable phosphor as described, for example, in U.S. Pat. No. 4,239,968 has been recently paid much attention. The method involves the steps of causing a stimulable phosphor to absorb a radiation having passed through an object or having radiated from an object; sequentially exciting (or scanning) the phosphor with an electromagnetic wave such as visible light or infrared rays (stimulating rays) to release the radiation energy stored in the phosphor as light emission (stimulated emission); photoelectrically detecting the emitted light to obtain electric signals; and reproducing the radiation image of the object as a visible image from the electric signals.
In the radiation image recording and reproducing method, a radiation image is obtainable with a sufficient amount of information by applying a radiation to the object at a considerably smaller dose, as compared with the conventional radiography. Accordingly, this method is of great value, especially when the method is used for medical diagnosis.
As a stimulable phosphor employable in the abovedescribed method, U.S. Pat. No. 4,239,968 discloses a rare earth element activated alkaline earth metal fluorohalide phosphor having the formula: EQU (Ba.sub.1-x,M.sup.2+.sub.x)FX:yA
in which M.sup.2+ is at least one alkaline earth metal selected from the group consisting of Mg, Ca, Sr. Zn and Cd; X is at least one halogen selected from the group consisting of Cl, Br and I; A is at least one element selected from the group consisting of Eu, Tb, Ce, Tm, Dy, Pr, Ho, Nd, Yb and Er; and x and y are numbers satisfying the conditions of 0.ltoreq.x.ltoreq.0.6 and 0.ltoreq.y.ltoreq.0.2, respectively.
The phosphor gives emission (stimulated emission) in the near ultraviolet region when excited to an electromagnetic wave such as visible light or infrared rays after exposure to a radiation such as X-rays.
The above mentioned rare earth element activated alkaline earth metal halide phosphor has been previsously known as a stimulable phosphor employable in the radiation image recording and reproducing method utilizing a stimulability thereof as described above, but almost no stimulable phosphor other than said phosphor has been known.
As a phosphor having the same alkali metal halide as host component as in the phosphor of the present invention, there has been previously known a thallium or sodium activated cesium iodide phosphor (CsI:Tl or CsI:Na). This phosphor gives emission (spontaneous emission) when exposed to a radiation such as X-rays, cathode rays and ultraviolet rays.